The conventional way of introducing a solid ingredient into a polymer is by melt-processing a mixture of the ingredient and polymer, either in an extruder, or by plasticizing the polymer sufficiently so as to be able to mix the ingredient into the plasticized mass. Such a procedure may be applicable if an ingredient can withstand a temperature at which the polymer is plasticizable, in the case of polyvinyl chloride (PVC), this temperature is in the range from about 180° C. to about 200° C.
When the properties of an ingredient are to be preserved when it is incorporated into a polymer, melt-processing an ingredient which is degradable at a temperature required for melt-processing is thus negated.
In the prior art, SC—CO2 has been used mainly to extract an organic ingredient from a substrate in which it is distributed, or to separate one organic compound from another. Each of the foregoing relies upon the known higher solvent power of a fluid as its density increases. A supercritical fluid is sufficiently dense to swell a polymer even if the polymer is essentially insoluble in the supercritical fluid, by virtue of forcing molecules of the supercritical fluid into the pores of the polymer, as taught by U.S. Pat. No. 5,340,614 to Perman et al., provided a carrier liquid was also used to “carry” the additive into the pores of the polymeric substrate. However, Perman et al were unconcerned with either uniformity of distribution of infused particles, or their size, and make no mention of either.
Unlike the foregoing, U.S. Pat. No. 4,820,752 teaches “infusing into a rubber or plastic polymeric material” (sic) any additive, liquid or solid, which has a “degree of solubility in said polymeric material when said polymeric material is in a swollen state”, using a normally gaseous fluid which could be compressed to supercritical conditions. Clearly, solubility of the additive in the polymer is required, the degree of solubility being at least 0.1 percent, and under the high pressure conditions described, any microporous polymer substrate will necessarily become swollen.
Moreover, it is not clear whether the fluid is required to dissolve either the additive or the polymer, or both, or simply swell the polymer, such equivocation being stated as follows: “A fluid may have sufficient solvent or swelling power to be useful in practicing this invention if sufficiently compressed at temperatures above, equal to, or below the critical temperature of the fluid” (see '752 patent, col 4, lines 45–49). Evidently the only requirement of the fluid to provide all the necessary properties to infuse a polymer with any additive is that the fluid be derived by compressing a normally gaseous fluid. Such evidence is provided in Examples 1 and 2 teaching the use of carbon dioxide at 22° C. and pressure of 59.6×105 Pa (or 5960 kPa), and in Examples 3 and 4 at 22° C. and pressure of 65.1×105 Pa (or 6510 kPa), under which conditions carbon dioxide is not supercritical. The supercritical conditions for carbon dioxide are 31.4° C. and 73.4 atm (7435.42 kPa). Only Example 6 deals with SC—CO2 which was used to impregnate a polyurethane sheet with progesterone, the solubility of which, in SC—CO2, or lack thereof, was not stated.
Still further, the '752 patent states: “In accordance with the present invention, an additive desired to be included in a rubber or plastic composition is dissolved in a compressed normally gaseous fluid.” ('752, col 2, lines 16–17) but requires only that the additive have a “degree of solubility” stated as follows: “The fluid and additive are chosen so that the additive has a degree of solubility in the polymer into which it is to be infused and so that the solution of fluid and additive has a degree of solubility in the polymer and is capable of swelling the polymer.” ('752, col 2, lines 29–33). If the “degree of solubility” included essentially complete solubility, then the polymer too would be dissolved in the “the solution of fluid and additive”.
As evidenced by illustrative example 1, “other sample chips were exposed in a pressure vessel to carbon dioxide in the presence of solid naphthalene at room temperature and a pressure of 59.6×105 pascals for 92 hours;” indicating that all the naphthalene was not in solution. This may be attributable to the fact that the carbon dioxide was not under supercritical conditions. The only example where SC—CO2 was used (Example 6) states that a sheet of polyurethane “was exposed to progesterone in the presence of carbon dioxide at a temperature of 45° C. and a pressure of 151.6×105 pascals for 4.5 hr” indicating not all the progesterone was in solution. Thus, Berens et al were unaware that solid particles of relatively large size could be transported if fully dissolved in SC—CO2 to form a solids-free solution, and the solids redeposited in a polymer substrate as micronized crystals.
Persons of ordinary skill in the art are well aware that the solubility of an additive in a supercritical fluid, and in SC—CO2 in particular, cannot be predicted. This is particularly true for inorganic salts. For example, as shown below, sodium nitrite is soluble in SC—CO2, but sodium chloride is not. In view of such unpredictability it should now be evident that a teaching that any normally solid additive, whether organic or inorganic, may be transferred from a liquid under pressure into a swellable polymer, as taught in the '752 patent, is at best overly broad.
U.S. Pat. No. 4,290,912 issued to Boerwinkle et al, about two decades ago, disclosed that an inorganic nitrite, and in particular, an alkali metal nitrite, e.g. potassium nitrite, sodium nitrite and calcium nitrite, in combination with a 2,4,6-trisubstituted phenol provided an effective volatile corrosion inhibitor (VCI) when distributed in a lower (C2–C5) polyolefin (PO) polymer. A specific '912 combination comprised about equal parts (1.485 phr each) by weight of sodium nitrite and a 2,4,6-tri-substituted phenol containing 9 to 24 carbon atoms, specifically 2,6-di-tert-butyl-4-methyl phenol, along with small amounts of one or more inert ingredients such as fumed silica and oleyl alcohol which are known to possess no anti-corrosive properties. Effectiveness of the '912 film was unconcerned with the primary particle size of the sodium nitrite because the '912 patent did not address the problems (i) of uniformity of distribution, (ii) of particle size, or (iii) of maintaining transparency of extruded film or molded articles having smooth surfaces.